There
are lots of different hot air solar collector designs to choose
from, but which is the best?

It
seems like an easy question. If my collector's output temperature
is hotter than yours, it must be better, right? Not so
fast! There are tons of folks, particularly on YouTube,
touting really high output numbers with their designs, but if you
blow more than a breath of air through their collectors, their output
temperatures might just fall like a rock!

Along
with temperature rise there is another, equally important variable.
It is the amount of air flowing through a collector, which
is usually measured in cubic feet per minute (CFM).

In
basic terms, If my collector is as hot as yours, but you have twice
the air flowing through your collector, yours is working twice as well!
If I raised my airflow to equal yours, my temperature increase
would only be half of what yours is.

Both
temperature rise and airflow are integral to comparing hot air collectors.
This is a really important concept to keep in mind. As
soon as someone tells you how hot their collector is, the first
thing you should be wondering is how much air they are flowing through
it. If it isn't much, the hot temperatures they are touting
don't mean much. This same principle applies to water collectors
too.

At
this point you may be thinking that as long as we measure our temperature
rise and adjust for airflow, it should be straightforward to compare
collector performances. Again, not so fast! We've accounted
for the two biggest variables, but by no means all of them. Here
are a few more:

-
Even on days that appear perfectly sunny, high, thin clouds
that are virtually invisible can vary the sun's intensity quite
a bit.- The outside temperature might be colder at my
house than yours, affecting the performance some.- The collectors might be at different
tilt angles or aren't quite facing the same direction, which also
affects the sun's intensity striking the collector.- It
might be windier at your house, pulling more heat off the glazing

The
only reliable way to determine the performance of one collector
to another is to compare them side by side under identical conditions.

Gary
Resa of www.builditsolar.com
in Montana and I, here in Maryland, set out to do just that in a
collaborative effort. Here is a picture of my test collector, consisting
of three, 4' X 8' bays. Each bay has an airtight separation
from the others and each is fed individually.

My
three bay test collector (screen not yet installed in Bay 1)

It
is absolutely amazing! Here we are in the 21st Century and
there is still tons of fertile ground for experimentation
by the solar hobbyist
/ enthusiast. There are many designs and materials to try
and the opportunity to learn and to contribute
to the art and science of DIY solar!

Enthusiasts in other areas of interests like backyard astronomy or Amateur Radio have been helping to move those disciplines forward at both the hobbyist and professional level for decades. Meanwhile, solar is equally fun, interesting, obviously needed, less expensive to pursue and actually returns your investment many times over; yet there are opportunities everywhere for the backyard solar enthusiast yet to explore! Plus, there are tax credits available too. While the tax legislature usually changes with each year, most personal tax prep software packages are automatically equipped to handle it. If you have an interest in experimenting with solar, please join us. Your ideas could make a difference on a scale bigger than you can possible imagine and you'll have a lot of fun along the way!

Before
my Recommendations - First, a Few Prefaces

We have hardly scratched the surface on our testing process. In fact,
we are still figuring out the best ways to perform the tests, much less
trying different variations of our current absorber types. Then we have
countless other types of materials to try. More
folks testing different designs or corroborating our tests would
be extremely helpful in moving the process forward!
So, you may be wondering
why I'm already offering some conclusions and recommendations. There
are a few reasons:

1. This testing process could take a lifetime for several
of us Right now, there are only two or three of us doing these tests.
If folks wait for the "final" answer, they'll never build anything. It's
like waiting to buy a computer until the processors quit getting better -
you'll never own one!

2. While we have a heck of a lot
of testing ahead, we have gleaned at
least some reasonable data on direct, comparative testing with four
different, popular, collector designs, - a backpass reference collector,
an empty box, a vented soffit and a fiberglass screen collector. In addition,
while we don't have side by side comparison numbers, we have very good
data on a 5th design - the aluminum downspout, compiled mostly by Scott
S and to a lesser degree myself. Also, we have some first hand
experience on building a variety of collectors and an estimate of their
costs. That's good, final data that helps with our conclusions at present.

3. I continue to get a lot of direct e-mail asking for performance data
updates and design recommendations from folks eager to get started on
their collectors. They wonder what I would recommend now, based on what
we've learned so far.

Current
Recommendations

Some
of you may be quite interested in the testing details and
I have included them below, but for those who are interested in
conclusions and recommendations so far, if someone were to ask me today
what type of hot air collector I would recommend building, I would
answer them this way:

For a traditional, 4' X 8' design, I would build a collector with a two or three layer
aluminum window screen.

- Best comparative performance
- Least expensive by far (a 25 foot roll of 4 foot wide, aluminum screen
is only about $29 at Home Depot). Fiberglass screen is even
cheaper and performs great, but we are uncertain about the paint
at really high temperatures.
- Easiest, quickest build by far
- Lowest pressure drop (least resistance to airflow besides a black
box) That means you can get a higher airflow for better efficiency than
you would experience with the same sized fan and other collector types.

For a long, low collector, I would build an aluminum downspout design.

- Good performer. We don't have side by side comparative
performance numbers, however, Scott S. did some very detailed
measurements and calculations that show the aluminum downspout design works
really well. You'll find complete construction details and
Scott's data documenting the performance at the bottom of the page here: http://www.n3fjp.com/solar/solarhotair.htm
- Very easy to build
- Downspout material lends itself to a long, low construction design.
This gives almost unlimited flexibility in design dimension
options.

Though it is also a good performer, I would steer folks away from the backpass design due to the extremely high pressure drop.

I would definitely steer folks away from the black box due to the poor comparative performance.

The vented soffit appears to be a very good performer and is also a
good choice. I would choose the screen, however, because the screen
appears to be performing slightly better, it is much cheaper, easier and
quicker to build.

So, there you have it. Based on what I know today, those are my recommendations.

We have tons to learn. Who knows what may come to light down the road,
but if you are planning to build a collector, don't wait. The screen
and downspout collectors are easy to build and they work great. In the mean time,
the longer you wait, the more days of sunshine go by before you ever
have a collector for them to shine on. Any
collector will work infinitely better than no collector!

What
about aluminum downspout collectors compared to screen?

Questions
come up all the time on how aluminum downspout collectors compare
to the screen collectors. The aluminum downspout collector
is a super design that has become very popular. There
have been lots of good reports on the downspout design, I think in
part because a downspout collector has lots of the ingredients of a
successful design:

- It keeps the heated air contained in the downspouts, well away from the glazing.
It doesn't mix with the air outside the downspouts inside
the collector at all

- There is no moving air at all near the glazing

- The downspout wraps all the way around the airflow, so there is a lot of heat transfer area for the air the rub against

-
It is very easy to seal so there is no outside air infiltration

I
haven't done any side by side testing of the downspout collector compared to
screen in my test collector. I thought about it, but then realized that
the configuration in the test collector would not be representative of
how folks are building the long low design with downspouts. Both designs
work
great, so I think the choice comes down to the dimensions of the
collector you plan to build. I'd opt for the downspout collector for a
long collector and screen for a tall collector.

Testing
Details

Gary
Resa of www.builditsolar.com
and I have been working on this project in a collaborative effort.
We would love to have you join us! Here is a
link to Gary's test details and results:

Here is a YouTube
Video summarizing the characteristics of a high performance, hot
air collector and our results so far:

:

Testing Premise
- Using a Reference Collector for Comparison:

As explained
above, there are lots of variables that make side by side testing
a challenge, but Gary and I wanted to come up with a way for folks
who are geographically separate from each other to be able to contribute
with meaningful, comparative data. We also wanted to have
a baseline to compare various designs at our own locations on different
days and inevitably different conditions.

What we decided
to do was to each build a baseline, reference collector that is
easily duplicated, so that the relative performance should be identical.
The reference collector would not ever be changed. Other
collectors would be run against the reference standard in side by
side tests, with the primary result - temperature rise - being compared.
In other words, if the reference standard raises the temperature
50 degrees and collector B raises the temperature 60 degrees with
the same air flow, collector B can be said to outperform the reference
standard by 20% (10 / 50).

Presently, we
are using a backpass design for the reference collector, documented
in detail on Gary's site. The backpass works well, but we
are considering the possibility of selecting a different reference
design, because the backpass requires a lot more pressure to move
air through than other designs.

Air Flow

With accurate
sensors, measuring the temperature entering and exiting the collector
is easy. Measuring airflow is another matter entirely. We
have tried bag tests, measuring the voltage of computer fans and
inserting an anemometer in the air stream. The bag test may
be the most accurate, but it wasn't an option for my configuration
here. These two short videos show how I'm balancing flow:

I
am also inserting a Kestrel anemometer in the air stream as a secondary
check on air flow.

Results
so Far

So
far, I have compared the reference backpass to the fiberglass screen
and vented soffit designs here. In addition, Gary has also
compared the black box and has data for that. Averaging measurements
over a two day period, here are my results for the backpass standard,
fiberglass screen and vented soffit:

So,
the two layer fiberglass screen collector is outperforming the
backpass by an average of 7.5%, giving it a clear advantage over
the reference backpass and vented soffit collector. In addition,
it was by far the easiest, quickest and least expensive collector
to make.

That's
where we are so far. I hope to update this page as additional
tests are performed. Right now, the fiberglass screen collector
is the heat absorber to beat. Do you think you can come up
with a design that can? I would love to see you do it! Bring
it on, we will all win!!!

If you have an interest in brainstorming and testing solar projects,
or you are brand new to solar and want some help getting started,
we would love to have you subscribe:

SimplySolar - Solar
Forum and E-mail Groups!

It
turns out that there are other folks like me who also enjoy
sharing ideas and learning from each other's experiments! If you have an interest in brainstorming
solar projects that are easy and inexpensive to build and
neighborhood friendly, or want some help with a project that you have
underway, please join us!

I originally set up an e-mail group - SimplySolar, for that purpose.
The e-mail group has served us well, but with overwhelming
growth and interest in the e-mail group, to better keep content
organized and give members the option to easily follow only the
threads that interest them, we have just set up a new Simply Solar
on line forum!
SimplySolar is about brainstorming and sharing ways to implement solar heat in
easy ways that the average homeowner, who may not be much of a
"do-it-yourselfer" (like me), can use to put money back in their pockets, green
back in the environment and have a lot of fun along the way! If
solar excites you, we would love to have you join our forum: